"This was a crucial step in preparing the aircraft for ground testing, engine runs, taxi tests, and ultimately first flight," said Jean Floyd, CEO of Stratolaunch, in a statement

The plane is the largest in the world based on wingspan, measuring 385 feet, which is longer than a NFL football field.

The Stratolaunch aircraft features two cockpits and six Boeing 747 engines. It's capable of carrying payloads up to 550,000 pounds.

Allen founded the company in 2011 as part of a plan to make travel to low Earth orbit more accessible.

"Opening up access to (low Earth orbit) will deliver many benefits," Allen wrote on LinkedIn last year. "For example, we could deploy more satellites that would enable better understanding of why our weather patterns are changing and help increase agricultural productivity. And, we could study atmospheric chemistry more closely to better study and mitigate climate change."

Floyd says the aircraft will start testing at the Mojave Air and Space Port over the coming months, with plans for a launch demonstration in 2019.

"This is a first-of-its-kind aircraft, so we’re going to be diligent throughout testing and continue to prioritize the safety of our pilots, crew and staff," said Floyd.